DEV: Add converter framework for migrations-tooling (#28540)

* Updates GitHub Actions
* Switches from `bundler/inline` to an optional group in the `Gemfile` because the previous solution didn't work well with rspec
* Adds the converter framework and tests
* Allows loading private converters (see README)
* Switches from multiple CLI tools to a single CLI
* Makes DB connections reusable and adds a new abstraction for the `IntermediateDB`
* `IntermediateDB` acts as an interface for IPC calls when a converter steps runs in parallel (forks). Only the main process writes to the DB.
* Includes a simple example implementation of a converter for now.

migrations/spec/lib/database/offline_connection_spec.rb

@@ -0,0 +1,67 @@
+# frozen_string_literal: true
+
+RSpec.describe ::Migrations::Database::OfflineConnection do
+  subject(:connection) { described_class.new }
+
+  let!(:sql) { "INSERT INTO foo (id, name) VALUES (?, ?)" }
+
+  it_behaves_like "a database connection"
+
+  describe "#close" do
+    it "removes the cached statements" do
+      connection.insert(sql, [1, "Alice"])
+      connection.insert(sql, [2, "Bob"])
+
+      expect(connection.parametrized_insert_statements).to_not be_empty
+
+      connection.close
+      expect(connection.parametrized_insert_statements).to be_nil
+    end
+  end
+
+  describe "#closed?" do
+    it "correctly reports if connection is closed" do
+      expect(connection.closed?).to be false
+      connection.close
+      expect(connection.closed?).to be true
+    end
+  end
+
+  describe "#insert" do
+    it "can be called without errors" do
+      expect { connection.insert(sql, [1, "Alice"]) }.not_to raise_error
+    end
+  end
+
+  describe "#parametrized_insert_statements" do
+    it "returns an empty array if nothing has been cached" do
+      expect(connection.parametrized_insert_statements).to eq([])
+    end
+
+    it "returns the cached INSERT statements and parameters in original order" do
+      connection.insert(sql, [1, "Alice"])
+      connection.insert(sql, [2, "Bob"])
+      connection.insert(sql, [3, "Carol"])
+
+      expected_data = [[sql, [1, "Alice"]], [sql, [2, "Bob"]], [sql, [3, "Carol"]]]
+      expect(connection.parametrized_insert_statements).to eq(expected_data)
+
+      # multiple calls return the same data
+      expect(connection.parametrized_insert_statements).to eq(expected_data)
+      expect(connection.parametrized_insert_statements).to eq(expected_data)
+    end
+  end
+
+  describe "#clear!" do
+    it "clears all cached data" do
+      connection.insert(sql, [1, "Alice"])
+      connection.insert(sql, [2, "Bob"])
+      connection.insert(sql, [3, "Carol"])
+
+      expect(connection.parametrized_insert_statements).to_not be_empty
+
+      connection.clear!
+      expect(connection.parametrized_insert_statements).to eq([])
+    end
+  end
+end